I'd say definitely the Incredible. I don't know about you, but I would want at least the chance of getting updated Android OS releases on a phone that I bought today, and I think that it is far more likely that the Incredible will get later Android releases than the Eris will (my guess is that HTC and Verizon bring the Eris to 2.1 and that's it.)
If the Droid wasn't so heavy and thick and didn't have the crap keyboard back in December, I would have bought the Droid, no doubt. For me thin and more pocketable was more important to me.
But, you should have the ability after the 29th to go in to a Verizon Wireless store and actually try both out and decide for yourself.